1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine of, for example, a motor vehicle, and, more particularly, to an exhaust gas purifying catalyst capable of efficiently purifying nitrogen oxides (NOx) by reduction, from exhaust gas that contains an amount of oxygen in excess of the oxygen amount required for complete oxidation of the reducing components of the exhaust gas, such as carbon monoxide (CO), hydrogen gas (H.sub.2), carbon hydride (HC) and the like.
2. Description of the Related Art
Many conventional motor vehicles employ three-way catalyst that purify exhaust gas by simultaneously oxidizing CO and HC and reducing NOx in exhaust gas at a theoretical air-fuel ratio (stoichiometric ratio). In a widely known three-way catalyst of this type, a honeycomb base member formed of, for example, cordierite, carries thereon a catalyst supporting layer formed of .gamma.-alumina, and the catalyst supporting layer supports catalytic noble metals such as platinum (Pt), rhodium (Rh) and the like.
Recently, carbon dioxide (CO.sub.2) in exhaust gas from internal combustion engines of motor vehicles and the like has become an issue in view of protection of global environments. A promising technology for reducing the amount of CO.sub.2 emission from an internal combustion engine is a lean burn system, in which combustion is performed at a lean air-fuel ratio with an excess amount of oxygen. The lean burn system reduces fuel consumption and, therefore, reduces the amount of CO.sub.2 produced by combustion.
Since the conventional three-way catalyst achieve simultaneous oxidation of CO and HC and reduction of NOx in exhaust gas when the air-fuel ratio is substantially the stoichiometric ratio, the conventional three-way catalyst fail to sufficiently remove NOx by reduction in lean-burnt exhaust gas, which contains an excess amount of oxygen. Therefore, there has been a need for development of a catalyst and an exhaust gas purifying system capable of purifying NOx even in an excess-oxygen(lean) atmosphere.
The present applicant proposed in, for example, Japanese Patent Laid-Open No. Hei 5-317652, an exhaust gas purifying catalyst in which an alkaline earth metal, such as barium (Ba) or the like, and platinum (Pt) are supported by a catalyst supporting layer formed of alumina or the like. Using the exhaust gas purifying catalyst, NOx can be efficiently removed from exhaust gas from a lean burn system if the air-fuel ratio is controlled so that the air-fuel ratio shifts from a lean side to a stoichiometric/rich side in a pulsed manner. NOx is adsorbed by the alkaline earth metal (NOx adsorber) on the lean side, and reacts with reducing components, such as HC, CO and the like, on stoichiometric/rich side.
To improve exhaust gas purifying performance, it is necessary to increase the probability of contact of emissions with the catalytic noble metal and the NOx adsorber. For this end, it is a main-stream practice to use active alumina, having a great specific surface area, to form a catalyst support layer. A normally employed base member is a honeycomb base member having a cell concentration of 400 cells/inch.sup.2, for an increased geometric surface area.
A greater absolute amount of the catalyst supporting layer is more favorable in order to sufficiently disperse the catalytic noble metal and the NOx adsorber. However, an increase in thickness of the catalyst supporting layer results in narrowed honeycomb channels and, therefore, an increased exhaust gas passage resistance. Therefore, in conventional exhaust gas purifying catalysts employing honeycomb base members with a cell concentration of about 400 cells/inch.sup.2, the amount of the catalyst supporting layer formed is normally set to about 120-240 g relative to 1 liter of the honeycomb base member.
In a normal method for forming a catalyst supporting layer on cell walls of a honeycomb base member, a base member is dipped into an aqueous slurry containing active alumina as a main component and, after the base member is pulled out of the slurry, extraneous slurry is removed from interiors of the cells by blowing from an rear end surface of the base member. The base member is then dried and baked, thereby forming a catalyst supporting layer.
In an exhaust gas purifying catalyst having a NOx adsorber, sulfur (S) contained in fuel is oxidized into SO.sub.2, which is further oxidized on the catalyst into SO.sub.3 or SO.sub.4, and these substances in turn react with the NOx adsorber, producing sulfuric acid salts of NOx adsorber. The NOx adsorbing capacity is thus lost, thereby impeding the removal of NOx by reduction. This undesired phenomena is termed sulfur-poisoning of NOx adsorber.
The sulfate of NOx adsorber can be re-generated into NOx adsorber by reduction at, for example, 650.degree. C. or higher in a rich or stoichiometric atmosphere before crystallization of the sulfate progresses. Once the crystallization progresses, however, the reduction of the sulfate becomes difficult even in a rich atmosphere, and the NOx purifying performance significantly decreases.